.\"	$NetBSD: execve.2,v 1.9 1995/02/27 12:32:25 cgd Exp $
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.\"     @(#)execve.2	8.3 (Berkeley) 1/24/94
.\"
.Dd January 24, 1994
.Dt EXECVE 2
.Os BSD 4
.Sh NAME
.Nm execve
.Nd execute a file
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fo execve
.Fa "const char *path"
.Fa "char *const argv[]"
.Fa "char *const envp[]"
.Fc
.Sh DESCRIPTION
.Fn execve
transforms the calling process into a new process.
The new process is constructed from an ordinary file,
whose name is pointed to by
.Fa path ,
called the
.Em new process file .
This file is either an executable object file,
or a file of data for an interpreter.
An executable object file consists of an identifying header,
followed by pages of data representing the initial program (text)
and initialized data pages.  Additional pages may be specified
by the header to be initialized with zero data;  see
.Xr a.out 5 .
.Pp
An interpreter file begins with a line of the form:
.Pp
.Bd -filled -offset indent -compact
.Sy \&#!
.Em interpreter
.Bq Em arg ...
.Ed
.Pp
When an interpreter file is
.Fn execve Ap d ,
the system runs the specified
.Em interpreter .
If any optional
.Em args
are specified, they become the first (second, ...) argument to the
.Em interpreter.
The name of the originally
.Fn execve Ap d
file becomes the subsequent argument;
otherwise, the name of the originally
.Fn execve Ap d
file is the first argument.
The original arguments to the invocation of the interpreter
are shifted over to become the final arguments.
The zeroth argument, normally the name of the
.Fn execve Ap d
file, is left unchanged.
.Pp
The argument
.Fa argv
is a pointer to a null-terminated array of
character pointers to null-terminated character strings.
These strings construct the argument list to be made available to the new
process.  At least one argument must be present in
the array; by custom, the first element should be
the name of the executed program (for example, the last component of
.Fa path ) .
.Pp
The argument
.Fa envp
is also a pointer to a null-terminated array of
character pointers to null-terminated strings.
A pointer to this array is normally stored in the global variable
.Va environ.
These strings pass information to the
new process that is not directly an argument to the command (see
.Xr environ 7 ) .
.Pp
File descriptors open in the calling process image remain open in
the new process image, except for those for which the close-on-exec
flag is set (see
.Xr close 2
and
.Xr fcntl 2 ) .
Descriptors that remain open are unaffected by
.Fn execve .
.Pp
Signals set to be ignored in the calling process are set to be ignored in
the
new process. Signals which are set to be caught in the calling process image
are set to default action in the new process image.
Blocked signals remain blocked regardless of changes to the signal action.
The signal stack is reset to be undefined (see
.Xr sigaction 2
for more information).
.Pp
If the set-user-ID mode bit of the new process image file is set
(see
.Xr chmod 2 ) ,
the effective user ID of the new process image is set to the owner ID
of the new process image file.
If the set-group-ID mode bit of the new process image file is set,
the effective group ID of the new process image is set to the group ID
of the new process image file.
(The effective group ID is the first element of the group list.)
The real user ID, real group ID and
other group IDs of the new process image remain the same as the calling
process image.
After any set-user-ID and set-group-ID processing,
the effective user ID is recorded as the saved set-user-ID,
and the effective group ID is recorded as the saved set-group-ID.
These values may be used in changing the effective IDs later (see
.Xr setuid 2 ) .
.Pp
The new process also inherits the following attributes from
the calling process:
.Pp
.Bl -column parent_process_ID -offset indent -compact
.It process ID Ta see Xr getpid 2
.It parent process ID Ta see Xr getppid 2
.It process group ID Ta see Xr getpgrp 2
.It access groups Ta see Xr getgroups 2
.It working directory Ta see Xr chdir 2
.It root directory Ta see Xr chroot 2
.It control terminal Ta see Xr termios 4
.It resource usages Ta see Xr getrusage 2
.It interval timers Ta see Xr getitimer 2
.It resource limits Ta see Xr getrlimit 2
.It file mode mask Ta see Xr umask 2
.It signal mask Ta see Xr sigaction 2 ,
.Xr sigsetmask 2
.El
.Pp
When a program is executed as a result of an
.Fn execve
call, it is entered as follows:
.Bd -literal -offset indent
main(argc, argv, envp)
int argc;
char **argv, **envp;
.Ed
.Pp
where
.Fa argc
is the number of elements in
.Fa argv
(the ``arg count'')
and
.Fa argv
points to the array of character pointers
to the arguments themselves.
.Sh RETURN VALUES
As the
.Fn execve
function overlays the current process image  with a new process image,
the successful call has no process to return to.
If
.Fn execve
does return to the calling process, an error has occurred;
the return value will be -1 and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn execve
will fail and return to the calling process if:
.Bl -tag -width Er
.\" ==========
.It Bq Er E2BIG
The number of bytes in the new process's argument list
is larger than the system-imposed limit.
This limit is specified by the
.Xr sysctl 3
MIB variable
.Dv KERN_ARGMAX .
.\" ==========
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.\" ==========
.It Bq Er EACCES
The new process file is not an ordinary file.
.\" ==========
.It Bq Er EACCES
The new process file mode denies execute permission.
.\" ==========
.It Bq Er EACCES
The new process file is on a filesystem mounted
with execution disabled
.Pf ( Dv MNT_NOEXEC
in
.Ao Pa sys/mount.h Ac ) .
.\" ==========
.It Bq Er EFAULT
The new process file is not as long as indicated by
the size values in its header.
.\" ==========
.It Bq Er EFAULT
.Fa Path ,
.Fa argv ,
or
.Fa envp
point
to an illegal address.
.\" ==========
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.\" ==========
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
This is taken to be indicative of a looping symbolic link.
.\" ==========
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 
.Dv {NAME_MAX}
characters, or an entire path name exceeded 
.Dv {PATH_MAX}
characters.
.\" ==========
.It Bq Er ENOENT
The new process file does not exist.
.\" ==========
.It Bq Er ENOEXEC
The new process file has the appropriate access
permission, but has an unrecognized format
(e.g., an invalid magic number in its header).
.\" ==========
.It Bq Er ENOMEM
The new process requires more virtual memory than
is allowed by the imposed maximum
.Pq Xr getrlimit 2 .
.\" ==========
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.\" ==========
.It Bq Er ETXTBSY
The new process file is a pure procedure (shared text)
file that is currently open for writing or reading by some process.
.El
.Sh CAVEAT
If a program is
.Em setuid
to a non-super-user, but is executed when
the real
.Em uid
is ``root'', then the program has some of the powers
of a super-user as well.
.Sh SEE ALSO
.Xr exit 2 ,
.Xr fork 2 ,
.Xr execl 3 ,
.Xr sysctl 3 ,
.Xr environ 7
.Sh HISTORY
The
.Fn execve
function call appeared in
.Bx 4.2 .
